Prosthetic feet are well known in the art, and several such feet have been designed to accomplish one or more objectives.
A useful prosthesis will simulate the operation and motion of an anatomical foot to a desired degree. An anatomical foot, including the ankle joint, is capable of motion around three perpendicular axes, as well as varying degrees of flexure. Specifically, the anatomical foot and ankle are capable of dorsiflexion, planiflexion, inversion, eversion, and transverse rotation. Dorsiflexion and planiflexion comprise the movement up and down of the ball of the foot with respect to the heel that occurs during a normal forward step. Inversion and eversion are the twisting of the foot around its longitudinal axis, resulting in outward and inward tilting of the ankles, respectively. Transverse rotation occurs when the foot rotates with respect to the longitudinal axis of the leg, such as occurs during left and right turns of the body.
Known foot prostheses include commercial feet that are capable of all three types of rotation. Typically, however, the joints capable of such complicated motion require bulky moving parts and are generally far too heavy for geriatric or very young patients, or other patients who suffer some degree of muscular weakness. Furthermore, some patients may not require high degree of flexibility, and may even desire the increased stability associated with a prosthesis that is capable of rotation about a single axis. Such a prosthesis would allow dorsiflexion and planiflexion, but not inversion, eversion, or transverse rotation. This would permit a near-normal step during forward motion, and would reduce the likelihood that the wearer might lose his or her balance as a result of uncontrolled or unintended rotation about one of the other axes.
In addition, it is desirable for a foot prosthesis to be capable of absorbing, storing, and releasing energy, so that the prosthesis returns itself to a relaxed, unflexed position when the moving force is removed. Prostheses that are designed for use during athletic activities, such as running or playing basketball, are particularly efficient at energy storage and return, providing a springy step. Such energy storage is typically accomplished by the inclusion of coil springs or other reciprocating means which absorb energy on flexure and release it efficiently upon removal of the applied force. The energy-storing components that are typically used for efficient energy return can contribute significantly to the weight of the prosthesis.
In contrast, older, less mobile wearers neither need nor want a high degree of return of stored energy. Instead, it is preferable for the prostheses worn by these wearers to absorb and dissipate a portion of the energy of each flexion while retaining sufficient energy to return the prosthesis to its unflexed position. This provides a more stable, cushioned step, and reduces the shock experienced by both the wearer and the prosthesis at each step.
Finally, it is necessary that a foot prosthesis be strong enough to support its wearer and durable enough to withstand the stresses of repeated stepping motions over long periods of time. Conventional prostheses tend to be designed for maximize strength, at the cost of added bulk and weight, making them unsuitable for geriatric or very young wearers, who do not subject their prostheses to the same loads as the average wearer.
Hence it is desired to provide a flexible, durable prosthesis that provides a slightly damped step and requires a minimal mass.